If an automatic transmission has trouble shifting gears, it could be because the transmission fluid is low, the fluid is the wrong viscosity or the throttle cables need adjusting. If these factors are not the problem, then it is best to take the vehicle to a licensed mechanic for service.
The first thing to check when an automatic transmission has trouble shifting is the transmission fluid. If it's low, one should refill it. If it's not the fluid, one should check if the fluid is the right thickness based on the owner's manual recommendation. If it's too thick, it prevents the fluid from flowing properly and will cause shifting problems. Finally, one should check the throttle cables. One should be sure the cables are properly secured and then test drive the vehicle again. If the vehicle still has a problem, it could be a much larger issue that requires service.